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People don't get flesh-eating bacteria very often, but it happens—and it's good to know it when you see it, LiveScience reports. A South Carolina man nearly lost an arm this week after contracting the infection while swimming, per WSPA, and USA Today reports on nine cases in Virginia this summer, including one fatality. One study found roughly four cases for every 100,000 Americans every year. That said, here's what you need to know:

People usually contract the ailment, known by doctors as necrotizing fasciitis, with a break in the skin that allows bacteria to enter. Think burns, scrapes, cuts, and surgical wounds, says the CDC.

Watch out for "pain that's out of proportion" with the wound, says one doctor. Patients may also notice crackling sensations or noises caused by air beneath the tissue.

At the outset, look for a red or swollen patch of skin surrounding the wound that expands faster than usual, and pain beyond the red patch. Such symptoms will begin within hours and should send you to a doctor right away.

If allowed to spread, the bacteria may cause confusion and even delirium. The pain might also subside as necrotizing fasciitis destroys the person's nerves, according to experts at the National Institute of Health.

Anyone can get flesh-eating bacteria, but it's often people with other health issues that diminish their power to stave off infection, like liver disease, cancer, kidney disease, or diabetes. Wounds exposed to seawater are also risky.

Anyone with necrotizing fasciitis should get aggressive treatment immediately. Mortality rates usually hover between 25% and 35%, but sometimes reach 50%.